Number of Instances of Stateless Session Bean

For a servlet(not SingleThreadModel) not hosted in a distributed environment(the default),the servlet container must use only one instance per servlet declaration.
All instances of a stateless session bean are equivalent.
In this case, why do the EJB container need to create more than one instances per stateless session bean?
thanks.

Would you like to explain why a servlet declaration
has only one instance?Handling the different client requests as separate threads in the service() method of the servlet is more efficient than using a pool of servlets. That's one of the reasons that the SingleThreadModel (usually implemented using a pool) is now deprecated, along with the fact that that model gives the developer a false sense of security as to multithreading issues...
Once we leave the Web tier to handle remote objects (EJBs) or database connections, we have a different problem, since we are faced with objects that are slow to create and tie up much more resources. In the biz and back-end tiers, using a pool of already existing instances gives actually better performance.

Similar Messages

Max number of instances for stateful session beans

Hi,
I am using OC4J 9.0.3 which is embedded in JDeveloper. I would like to set the maximum number of instances for stateful session beans. I know that the session-deployment section of the orion-ejb-jar.xml file has an attribute called max-instances, but it only works for stateless session beans. (at least it says so in the documentation and it did not work with my stateful session bean)
Anyone knows how to do it?
Thanks,
Leonardo Penha

Leonardo -- Stateful session beans do not come from a pool but are assigned 1 bean to 1 user. When the EJBs life ends the bean goes away. If what you are asking about is passivation configuraiton then that is different. We currently do not support passivation based on some count of SFSBs that exists inside the container but we will provide that capability in the next release of OC4J. (Note: When passivation occurs is mostly up to the vendors and it is not required by the J2EE 1.3 specification based on performance metrics. What is required is that when a SFSB is passivated it is done correctly and we do that.)
Thanks -- Jeff

EJB 3.1new instance of stateless session bean not created on new invocation

I am using Netbeans 6.8 bundle that comes with JAVA EE 6. I have created a web application then created restful web services and then created few stateless session beans (with local interfaces) that are invoked from restful web services. From the browser, when I call the url with get method, restful web services are being invoked and they in turn calls stateless session beans and does the appropriate business logic. While testing, I observed that for every new url call from the browser, the same bean is being invoked. I tested this by adding a instance variable vector and adding one item ('test") to the vector in bean's method. My understanding is that on every bean's method invocation the vector should have only one item. However, the vector is growing with many "test" items. I am literally confused, not sure this is the way it is supposed to work. I tried to invoke bean's method from restful web service by using both dependency injection and jndi look up, both instances the vector is growing with many "test" items.
I would really appreciate if some body gives me more insight.

I appreciate your answers. There is a strong reason for posting this question. Basically, I am using JPA and using stateless session beans to invoke database calls. In the bean I am using the following code for entity manager.
@PersistenceContext(unitName = "AsgProtocolServerPU")
private EntityManager em;
I have noticed very strange scenario while getting records. From the application, I have obtained few records of a table. Then, I manually deleted few records from the table. Very strangely, I have obtained the deleted records during my next call to bean. After 2 hours, I tested the same and did not get those records. This is not consistent.
I am totally confused. I am just wondering this is happening because I made beans local (not remote) and placed them in the web application (Java EE6 allows this). I am not sure whether this is JPA related issue or beans issue.

How Stateless Session Bean instances in pool

Hi,
I am not clear about , stateless bean having "pool" of objects to be used by multiple clients.when those instances will be created or who will generate them.
Is this pool specific to ejb container or any vendor container.
thanks in advance,

Hi , my question is about Stateless Session Bean ,
how StatelessSession bean will behave when some 100
clients wil call homeObject simultaneously.The EJBs(including stateless) are single threaded so one client can access one Bean at a time. As per your question if some 100 clients are invoking home.create() method then it has to invoke on 100 EJBs. The instance pool size can be specified while deploying. In the descriptor (application server speific). So if the number of simultaneous clients is more than the pool size then some clients have to wait.
Now my second question is how and who will create
these instance .The container will create the pool as per the application server spcific deployment descriptor.

EJB: Stateless Session Bean create() Question.

Lets say I have a stateless session bean that fetches data from my database. The point of the bean is to just do large SQL searches and funnel data back to the client. The prolem I have is that I am somehow fighting memory leaks. Despite having checked the code a number of times, the memory usage on my appserver continues to climb no matter what I do. I have theorized that the problem might be in the way Im using ma DataFetchBean (DFB).
When I start the client, he obtains a user session. This is a stateful session bean that he uses for almost all communication with the server. Then I call "getDataFetchBean" in the user session which calls DataFetchBeanHome.create(). Then the client holds onto the returned reference, using it for the live of his connection. As he disconnects, he calls remove on the bean.
Question is this:
1) Is it better for me to call create() prior to each call to the stateless session bean ?
2) Do you have any theories on why im loosing memory with this setup?
TIA
-- Rob

1. But I thought you were using a 'stateful session bean'?
2. For stateless session beans, there is no direct link between a remote reference and an instance of the bean. It is safe to hang on to the remote reference as long as you would like, of course it may go stale if the server dies. You will also find that the create() method does not actually contact the server, so doing it each usage costs very little. So, either way you should be fine.
3. As for memory leaks, make sure that you are closing all statements, result sets, etc. promptly. These are commonly the problem. Also, use hprof or some other profile tool to determine what types of data you are allocating and (with better tools) what types of data you may be holding on to references to.
Chuck

Singleton stateless session bean

hello
how to implement "singleton stateless session bean"?
thanks in advance!!

This is a case of premature optimization. Try forgetting about how to solve a singleton problem for the time being. Focus on the use-cases of your application and getting something working.
After that, if memory usage continues to be an issue, consider the following. If you have a large amount of data needed by multiple clients, one option to store it in a database and retrieve portions on demand. If you need to keep everything in memory in each bean instance you can also configure the total number of stateless session bean instances in your vendor's bean pool to limit the total resource usage. A third alternative is to use a third-party caching implementation or a vendor-specific singleton service.
--ken

Pl Clear my doubts on Stateless session Bean

Is it necessry to Have isolation level and transaction attribute to be mentioned Stateless session Bean.
and where this to be done.
Help is highly appreciated

There are default (leaving transaction state alone) - so you don't have to specify it.
I'm open to correction: isolation level is specified by the vendor specific configuration whereas transaction attributes are defined in ejb-jar.xml
isolation level is a tuning parameter, whereas the transaction attribute is a design issue.
You shouldn't specify transaction where they don't need. I have seen a number of applications where full transaction level is specified on all methods, and then they wonder why their application is slow.
A transaction is only required if atomic operations are in place. For instance, you don't need transactions on a getter, you may need them though on a setter, iff the setter does more than one thing and those things depend on each other.

WLS10 and Stateless Session Bean

I tried to create EJB3 application example.
1. Created Stateless Session Bean that implements Remote and Local interfaces:
Session Bean code:
package com.session;
import javax.ejb.Local;
import javax.ejb.Remote;
import javax.ejb.Stateless;
import javax.ejb.TransactionAttribute;
import javax.ejb.TransactionAttributeType;
@Stateless(mappedName="SessionBeanService")
@Remote(ISessionBeanRemote.class)
@Local(ISessionBeanLocal.class)
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public class SessionBean implements ISessionBeanLocal,
ISessionBeanRemote
public String reply(){
return "MySessionBean - success !!!";
Remote Interface code :
package com.session;
public interface ISessionBeanRemote
public String reply();
Local Interface code:
package com.session;
public interface ISessionBeanLocal
public String reply();
application.xml:
<?xml version="1.0" encoding="UTF-8"?>
<application xmlns="http://java.sun.com/xml/ns/javaee"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
          http://java.sun.com/xml/ns/javaee/application_5.xsd"
     version="5">
     <display-name>EJB3 Sample Application</display-name>
     <module>
     <ejb>beans.jar</ejb>
     </module>
</application>
weblogic-application.xml:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE weblogic-application PUBLIC
     "-//BEA Systems, Inc.//DTD WebLogic Application 8.1.0//EN"
     "http://www.bea.com/ns/weblogic/90/weblogic-application.xsd">
<weblogic-application>
          <classloader-structure>
               <module-ref>
                    <module-uri>beans.jar</module-uri>
               </module-ref>
          </classloader-structure>
</weblogic-application>
2. I packaged classes into EAR file and deployed to WLS10.
I didn't include any weblogic specific XML descriptors besides weblogic-application.jar.
My client code lookes as follows:
public void test(){
Context context = getMyServerContext();
// THIS JNDI NAME I SEE ON MY SERVER JNDI TREE
String jndiName = "sessionbeansbeans_jarSessionBean_ISessionBeanRemote";
Object obj;
obj = context.lookup(jndiName);
System.out.println(" obj class : " + obj.getClass().getName());
ISessionBeanRemote remote = (ISessionBeanRemote) PortableRemoteObject.narrow(
obj, ISessionBeanRemote.class );
String res = remote.reply();
System.out.println("res : "+res);
I get an Exception:
Exception occurred!
java.lang.ClassCastException: Cannot narrow remote object to com.session.ISessionBeanRemote
     at weblogic.iiop.PortableRemoteObjectDelegateImpl.narrow(PortableRemoteObjectDelegateImpl.java:242)
     at javax.rmi.PortableRemoteObject.narrow(PortableRemoteObject.java:137)
     at ca.cgi.mvest.test.server.wms.GlFacadeTest.runTest(GlFacadeTest.java:91)
     at ca.cgi.mvest.test.server.wms.GlFacadeTest.<init>(GlFacadeTest.java:53)
     at ca.cgi.mvest.test.server.wms.GlFacadeTest.main(GlFacadeTest.java:151)
java.lang.ClassCastException: Cannot narrow remote object to com.session.ISessionBeanRemote
My server console have the following output:
Root cause of ServletException.java.lang.NoClassDefFoundError: com/session/SessionBean_7pp7ls_ISessionBeanRemot
eIntf
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:12
4)
at weblogic.utils.classloaders.GenericClassLoader.defineClass(GenericCla
ssLoader.java:338)
at weblogic.utils.classloaders.GenericClassLoader.findLocalClass(Generic
ClassLoader.java:291)
Truncated. see log file for complete stacktrace
Server logs a problem already on the line when I do lookup
on JNDI name even before narrow();
It looks like my EAR was missing something. But server never complained during deployment.
May be someone can direct me to a real sample of Weblogic10-ejb3.0 application, since examples that come with WLS 10 intallation combersome and do not follow
docmentation.
Thanks in advance for any suggestion.

Hello Freind
The main different b\w stateful and statless is that stateful maintain state of method conversation means it has record that which method call before this method
but in case of stateless conversation state does not saved second different we can say that create method in stateless having no parameter but statefull having parameter I think u can understand easily
With Best Regards
Rajesh Pandey
email :[email protected]
url :-- http://www.sixthquadrant.com
Mob :-- 9811903737
Delhi India

Stateless Session Bean calling another Stateless Session Bean

Hi all,
I have written 2 Stateless Session bean and would like to call the business method of 2nd Stateless Session Bean in the business method of the 1st.
I have written the lookup code for 2nd SB in the business method of 1st SB and would like to execute the business method in the same method itself.
What do I have to do for the same for the following :
Deployment Descriptor file or any other file needs to be modified.
I am using Weblogic 6.1 and SQL Server 7.0 as database.
Thanks in advance,
Seetesh

This is actually quite similar to calling a session bean from anywhere.
Try this
//Session Bean 2 - Business Method, JNDI name SessionBean2
public void doSomethingMore(String what)
// Session Bean 1 which call.
public void doSomething()
   InitialContext initCtx = new InitialContext();
   SessionBean2Home home2 = null;
   SessionBean2 bean2 = null;
   try
     home2 = (SessionBean2Home)initCtx.lookup("SessionBean2");
     bean2 = home2.create();
   }catch(FinderException ex){..}
    catch(CreateExceptrion ex){..}
   try
     bean2.doSomethingMore("Anything");
   }catch(remoteException ex){..}
}In my case (webLogic 5.1) I did not need to change anything in any of the deployment descriptor. However, both beans needs top be deployed on same instance of Application server (might work across instances but didnt try)
This should do it.
Amitabh

Transaction management in stateless session beans.

Hi all,
I am using EJB 1.1.
I have a statless session bean that has two methods- A and B.
which does not involve any database interaction
like inserting/updating/deleting the data in the database.
The process flow is such the client always calls A first followed by the call to B.
I have the Transaction attribute set as TX_REQUIRED at the whole bean level.
Now my question is as follows:
Since it is a stateless bean, ejbCreate() is called for every method's invocation.
So does it mean that a new transaction is started for every method invocation?
Also since a transaction ends by commit/rollback.
The transation associated with the method A/B will never get completed as there is no commit/rollback involved in method implementation.
So how is this transaction ended?
Any more details about the transaction management in stateless session beans is highly appreciated.
Any input at the earliest is highly appreciated.
Thanks in advance.

Since it is a stateless bean, ejbCreate() is called for every method's invocation.For stateless session bean , Create() is not delegated to the instance.
So does it mean that a new transaction is started for every method invocation?This depends opon the Tx attribute and sequence of calls. Since you have given Tx_required then if you call any method and there is no Tx context associated with client,then a new TX will be started by container othere wise it will execute in the same TX context as the calling client. Note that client can be jsp or other ejb method.
Also since a transaction ends by commit/rollback.
The transation associated with the method A/B will never get completed >as there is no commit/rollback involved in method implementation.
So how is this transaction ended?If you are using COntainer managed TX then Transaction handling like starting , ending etc is handled by the container. You need not worry about that.
Any more details about the transaction management in stateless session >beans is highly appreciated.
Any input at the earliest is highly appreciated.Some time back I had read the article on how the Transaction management done by container on IBM Site. I dont have URL , but you can search the site.
HTH
-Ashwani

Paradox - Should entity CMP EJB be faster than stateless session bean?

Please bear with me on this.
Just thinking of this scenario: hello world application. It can be implemented both as stateless session bean and entity bean.
There's no persistent. So for the entity bean, it will be container managed, and no code to do anything to the load/remove/create. - comparably the same as stateless bean.
Next, for the stateless session bean, each client call would require a dedicated instance of the EJB on the server (although for a very short amount of time). For the entity bean, it's shared, so only 1 instance is needed. It's also thread safe.
So, for a very high traffic, shouldn't the entity bean do better because of single instance?
What's other overhead?
Does the entity bean cost more connections than the stateless bean?

Feels like a comparison between apples and oranges.
A stateless session bean is like every other instance, so these can be pooled by the EJB container. The container is free to create an instance for each client if necessary. That's different.
A stateless session bean doesn't have to deal with a database that may or may not be located on another machine the way an entity bean does. There could be another network round trip involved. That's different.
Session and entity beans are intended for entirely different purposes. Just because you can write Hello World with both doesn't mean it's representative of what EJBs are used for. I don't think it's a good, meaninful comparison. JMO - MOD

A tough one for EJB experts - Stateless session bean spec question

I am busy learning more about EJBs and came across something confusing regarding the legal operations in the various container callback methods for stateless session beans.
Specifically, the EJB spec states that in the ejbCreate() method, the SessionContext can be used to obtain a reference to the EJB Object. Now this makes perfect sense with stateful session beans, since the ejbCreate() method isn't called until a client is creating a bean and the container has linked that bean to the client's EJB Object. However, it is my understanding that when it comes to stateless session beans, the container creates the beans and adds them to the bean pool at its leisure. It is not until a business method is called by a client that a stateless bean is actually linked to an EJB object. So, how is it that a stateless bean could ever obtain a reference to an EJB Object from within ejbCreate(). Which EJB Object would it be linked to? This operation just doesn't appear to make sense in that context.
Can anyone clarify this for me?

Interesting question. I have such questions all the time! Here's a link to a similar discussion
http://saloon.javaranch.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic&f=70&t=000905
Also, I tried this using Weblogic 8.1. Tried to access the EJBObject in ejbCreate before any business method was invoked. I did this by specifying a value for initial-beans-in-free-pool and found that the hash code for the EJBObject was the same for all the bean instances that were created on startup.
I then invoked a business method and accessed the EJBObject in that method. Again the hash code for the object was the same as the one created on startup.
Seemed to be that there is a 1:n relation between the EJBObject and bean instances.
This may be container specific. The spec says the user should be able to invoke the getEJBObject() method in ejbCreate(), its upto the container to comply with it.

A question about entity manager in stateless session bean.

JSR 220 ejbcore, page 47 : stateless session bean: All business object references of the same interface type for the same stateless session bean have the "same object identity", which is assigned by the container.
So, if we have two session beans in client code...
@EJB Cart cart1;
@EJB Cart cart2;
then cart1.equals(cart2)==true
If we declare entity manager in stateless session bean:
@PersistenceContext( unitName="ds" ,type=PersistenceContextType.TRANSACTION)
private EntityManager em;If cart1 and cart2 are the same reference, do we have any problem when using the same reference(maybe the same em? ) to get data from db?

If cart1 and cart2 are the same reference, do we have
any problem when using the same reference(maybe the
same em? ) to get data from db?No. In EJB, there is a distinction between the EJB reference and the bean instance.
Each time you make an invocation on an EJB reference for a stateless session bean,
the container can choose any instance of that bean's bean class to process the
invocation. That's true whether you invoke the same reference multiple times or
two difference references to the same bean.
Each bean instance is guaranteed to be single-threaded.

EJBRemove on stateless session bean

I would like to create a stateless session bean that loads data from
the DB when it is created. Since the data being loaded is subject to
change, I'd like to make sure that the stateless session bean is
"destroyed" at least once every x seconds. Is it possible to do that
through the deployment descriptor ?
Mathieu
(No need to tell me that this bean might use data that is not in sync
with the db, I know that)

I still do not understand what is wrong with read-only entity beans - it
sounds like they do exactly what you want, automatically.
Also, stateless session beans have no identity and remove() doesn't do
anything.
Mathieu <[email protected]> wrote:
Let me be more specific:
I need to access some data (=> several rows from the DB) very
frequently. I don't need that data to be always up-to-date, I just
need to make sure that it is refreshed once in a while. Therefore, I
prefer to have to retrieve one session bean than X entity beans (one
per row). The argument about the expense of destroying the bean
doesn't seem to me to be a valid one because if I use read-only entity
beans, I would expect them to be destroyed and reloaded as well. And
furthermore, I don't understand why it would be costly to destroy such
a stateless session bean.
"David Shiffman" <[email protected]> wrote in message news:<3c130d6a$[email protected]>...
Agree. Even if it's possible to destroy your stateless bean on a timely basis this
approach is not the best for a number of reasons including the expense of destroying
the bean...
Dimitri Rakitine <[email protected]> wrote:
Hrm. Why don't you use ReadOnly entity beans for this purpose ?
Mathieu <[email protected]> wrote:
I would like to create a stateless session bean that loads data from
the DB when it is created. Since the data being loaded is subject to
change, I'd like to make sure that the stateless session bean is
"destroyed" at least once every x seconds. Is it possible to do that
through the deployment descriptor ?
Mathieu
(No need to tell me that this bean might use data that is not in sync
with the db, I know that)--
Dimitri
Dimitri

Looking up of a stateless session bean from an MDB

Hi,
I have been trying to invoke a method on a stateless session bean from an MDB on receipt of a message from a queue. The bean receives the message. Then it gets the initial context, but after this the following message appears:
javax.naming.NamingException: Error instantation environment context for Message DrivenBean MyMDB: No location specified and no suitable instance of the type 'MySessionRemote' found for the ejb-ref <mySessionBean> at com.evermind.server.ejb.MessageDrivenHome.getEnvironmentContext(MessageDrivenHome.java:559)
The session bean is in a seperate ear file. These are the deployment descriptors of the MDB:
ejb-jar:
<ejb-jar>
     <description>A Message-Driven Beans</description>
     <enterprise-beans>
     <message-driven>
          <description></description>
          <ejb-name>MyMDB</ejb-name>
          <ejbclass>MyMDB</ejb-class>
          <message-driven-destination>
          <jms-destination-type>javax.jms.Queue</jms-destination-type>
          </message-driven-destination>
          <ejb-ref>
          <ejb-ref-name>MySessionEJB</ejb-ref-name>
          <ejb-ref-type>Session</ejb-ref-type>
          <home>MySessionEJBHome</home>
          <remote>MySessionRemote</remote>
          </ejb-ref>
     </message-driven>
     </enterprise-beans>
     <assembly-descriptor>
     </assembly-descriptor>
</ejb-jar>
orion-ejb-jar
<orion-ejb-jar>
<enterprise-beans>
          <message-driven-deployment name="QueueProcessor" destination-location="jms/theQueue" connection-factory-location="jms/theQueueConnectionFactory">
          </message-driven-deployment>
     </enterprise-beans>
     <assembly-descriptor>
     </assembly-descriptor>
</orion-ejb-jar>
The lookup name which I give in MDB is:
java:comp/env/MySessionEJB

Hi,
    Thanks for your replies.I did as you had suggested.I added ejb-local-ref to the ejb-jar.xml and i provided ejb-ref as MyBean.
My lookup code uses the string
ctxt.lookup("localejbs\MyBean");
But still i get Naming Exception.
I tried something different yesterday.
I changed the code to use
InitialContext ctxt = new InitialContext();
ctxt.lookup("localejbs/"+ pathfromJNDIRegistry);
and this time i did not get Naming exception but i got RemoteException saying there was error loading the class.
Is there something i need to add somewhere for the classloader to be able to find and load this class in the second scenario.
Please suggest me which method to go for amongst the two and what is the missing information i need to add.
Thanks
Priya

Number of Instances of Stateless Session Bean

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